III-V Group Compound Devices with Improved Efficiency and Droop Rate

ABSTRACT

A photonic device includes: a first-type III-V group layer; a second-type III-V group layer formed on the first-type III-V group layer; and a multi-quantum well layer disposed between the first-type III-V group layer and the second-type III-V group layer; wherein: the multi-quantum well layer comprises a plurality of active layers interleaved with a plurality of barrier layers such that each barrier layer is separated from adjacent barrier layers by a respective one of the active layer; a material of each barrier layer comprises semiconductor compound devoid of Al element; the barrier layers comprises a first group layers between the first-type III-V group layer and the second-type III-V group layer and a second group layers between the second-type III-V group layer and the first group layers, and a thickness of each barrier layer of the first group layers is greater than that of each barrier layer of the second group layers; and the barrier layers of the first group layers comprise uniform thickness.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending application Ser. No.13/616,299, filed on Sep. 14, 2012, for which priority is claimed under35 U.S.C. §120; the entire contents of all of which are herebyincorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to III-V group compounddevices, and more particularly, to improving the efficiency and drooprate of III-V group compound devices such as gallium nitride (GaN)devices.

BACKGROUND

The semiconductor industry has experienced rapid growth in recent years.Technological advances in semiconductor materials and design haveproduced various types of devices that serve different purposes. Thefabrication of some types of these devices may require forming one ormore III-V group compound layer on a substrate, for example forming agallium nitride layer on a substrate. Devices using III-V groupcompounds may include light-emitting diode (LED) devices, laser diode(LD) devices, radio frequency (RF) devices, high electron mobilitytransistor (HEMT) devices, and/or high power semiconductor devices. Someof these devices, such as LED devices and LD devices, involve forming aquantum well having multiple pairs of active layers and barrier layers.The quantum well generates light when a voltage is applied. However,traditional LED and LD devices have quantum wells that have poorelectron-hole recombination, thereby leading to reduced output power andlarge efficiency droop for the LED and LD devices.

Therefore, while existing LED and LD devices have been generallyadequate for their intended purposes, they have not been entirelysatisfactory in every aspect. LED and LD devices having betterelectron-hole recombination continue to be sought.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1-3 are diagrammatic fragmentary cross cross-sectional side viewsof example LED structures according to various aspects of the presentdisclosure.

FIGS. 4A-4D, 5A-5D, and 6 are plots illustrating experimental dataaccording to various aspects of the present disclosure.

FIG. 7 is a diagrammatic fragmentary cross-sectional side view of anexample LED lighting apparatus according to various aspects of thepresent disclosure.

FIG. 8 is a diagrammatic view of a lighting module that includes the LEDlighting apparatus of FIG. 7 according to various aspects of the presentdisclosure.

FIG. 9 is diagrammatic fragmentary cross cross-sectional side views ofan example LD structures according to various aspects of the presentdisclosure.

FIG. 10 is a flowchart illustrating a method of fabricating a multiplequantum well for an LED or an LD according to various aspects of thepresent disclosure.

DETAILED DESCRIPTION

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of variousembodiments. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second features, such that the first and second features may not bein direct contact. Moreover, the terms “top,” “bottom,” “under,” “over,”and the like are used for convenience and are not meant to limit thescope of embodiments to any particular orientation. Various features mayalso be arbitrarily drawn in different scales for the sake of simplicityand clarity. In addition, the present disclosure may repeat referencenumerals and/or letters in the various examples. This repetition is forthe purpose of simplicity and clarity and does not in itself necessarilydictate a relationship between the various embodiments and/orconfigurations discussed.

As semiconductor fabrication technologies continue to advance, III-Vgroup compounds (also referred to as III-V family compounds or groupIII-V compounds) have been utilized to produce a variety of devices,such as light-emitting diode (LED) devices, laser diode (LD) devices,radio frequency (RF) devices, high electron mobility transistor (HEMT)devices, and high power semiconductor devices. A III-V compound containsan element from a “III” group (or family) of the periodic table, andanother element from a “V” group (or family) of the periodic table. Forexample, the III group elements may include Boron, Aluminum, Gallium,Indium, and Titanium, and the V group elements may include Nitrogen,Phosphorous, Arsenic, Antimony, and Bismuth.

Some of these III-V group compound devices, such as LEDs and LDs,contain a quantum well for emitting radiation such as different colorsof light in a visible spectrum, as well as radiation with ultraviolet orinfrared wavelengths. Compared to traditional light sources (e.g.,incandescent light bulbs), LEDs and LDs offer advantages such as smallersize, lower energy consumption, longer lifetime, variety of availablecolors, and greater durability and reliability. These advantages, aswell as advancements in LED and LD fabrication technologies that havemade LEDs and LDs cheaper and more robust, have added to the growingpopularity of LEDs and LDs in recent years.

Nevertheless, existing LEDs and LDs may have certain shortcomings. Onesuch shortcoming is that the quantum well for existing LEDs and LDs mayhave poor electron-hole recombination, thereby leading to reduced poweroutput and a large efficiency droop.

According to various aspects of the present disclosure, described belowis a photonic device having a quantum well with improved electron-holerecombination so as to increase output power and reduce the efficiencydroop associated with existing LEDs and LDs. In some embodiments, thephotonic device includes a horizontal LED. In some embodiments, thephotonic device includes a vertical LED. FIGS. 1 to 3 are diagrammaticcross-sectional side views of a portion of the LEDs at variousfabrication stages. FIGS. 1 to 3 have been simplified for a betterunderstanding of the inventive concepts of the present disclosure.

Referring to FIG. 1, a horizontal LED 30 is illustrated. The horizontalLED 30 includes a substrate 40. The substrate 40 is a portion of awafer. In some embodiments, the substrate 40 includes a sapphirematerial. In other embodiments, the substrate 40 may include a siliconcarbide material or a silicon material. The substrate 40 may have athickness that is in a range from about 50 microns (um) to about 1000um. In some embodiments, a low temperature buffer film may be formedover the substrate 40. For reasons of simplicity, however, the lowtemperature buffer film is not illustrated herein.

An undoped semiconductor layer 50 is formed over the substrate 40. Theundoped semiconductor layer 50 is free of a p-type dopant or an n-typedopant. In some embodiments, the undoped semiconductor layer 50 includesa compound that contains an element from the “III” group (or family) ofthe periodic table, and another element from the “V” group (or family)of the periodic table. In the illustrated embodiments, the undopedsemiconductor layer 50 includes an undoped gallium nitride (GaN)material.

The undoped semiconductor layer 50 can also serve as a buffer layer (forexample, to reduce stress) between the substrate 40 and layers that willbe formed over the undoped semiconductor layer 50. To effectivelyperform its function as a buffer layer, the undoped semiconductor layer50 has reduced dislocation defects and good lattice structure quality.In certain embodiments, the undoped semiconductor layer 50 has athickness that is in a range from about 1 um to about 5 um.

A doped semiconductor layer 60 is formed over the undoped semiconductorlayer 50. The doped semiconductor layer 60 is formed by an epitaxialgrowth process known in the art. In the illustrated embodiments, thedoped semiconductor layer 60 is doped with an n-type dopant, for exampleCarbon (C) or Silicon (Si). In alternative embodiments, the dopedsemiconductor layer 60 may be doped with a p-type dopant, for exampleMagnesium (Mg). The doped semiconductor layer 60 includes a III-V groupcompound, which is gallium nitride in the present embodiment. Thus, thedoped semiconductor layer 60 may also be referred to as a doped galliumnitride layer. In some embodiments, the doped semiconductor layer 60 hasa thickness that is in a range from about 2 um to about 6 um.

A pre-strained layer 70 is formed on the doped semiconductor layer 60.The pre-strained layer 70 may be doped with an n-type dopant such asSilicon. The pre-strained layer 70 may serve to release strain andreduce a quantum-confined Stark effect (QCSE)—describing the effect ofan external electric field upon the light absorption spectrum of aquantum well that is formed thereabove (i.e., the MQW layer 80 discussedbelow). In some embodiments, the pre-strained layer 70 contains a singleInxGa1-xN layer, where x is greater than 0 but less than 0.25. In someother embodiments, the pre-strained layer 70 includes a super-latticestructure. For example, the super-lattice structure may containInxGa1-xN (where x is greater than 0 but less than 0.2) and GaN layers.The pre-strained layer 70 may have a thickness in a range from about 30nanometers (nm) to about 200 nm.

A multiple-quantum well (MQW) layer 80 is formed over the pre-strainedlayer 70. The MQW layer 80 includes a plurality of alternating (orperiodic) active and barrier layers (also referred to as sub-layers).The active layers include indium gallium nitride (InGaN), and thebarrier layers include gallium nitride (GaN). For the sake of providingan example, seven barrier layers 81-87 and six active layers 91-96 areillustrated in FIG. 1 as an example MQW layer 80. The barrier layers81-87 and the active layers 91-96 are disposed in an interleavingmanner. The various aspects of the barrier layers 81-87 and the activelayers 91-96 will be discussed in more detail below.

An electron blocking layer 100 may optionally be formed over the MQWlayer 80. The electron blocking 100 layer helps confine electron-holecarrier recombination within the MQW layer 80, which may improve quantumefficiency of the MQW layer 80 and reduce radiation in undesiredbandwidths. In some embodiments, the electron blocking layer 100 mayinclude a doped aluminum gallium nitride (AlGaN) material, and thedopant may include a p-type dopant such as Magnesium. The electronblocking layer 100 may have a thickness in a range from about 15 nm toabout 30 nm.

A doped semiconductor layer 110 is formed over the electron blockinglayer 100 (and thus over the MQW layer 80). The doped semiconductorlayer 110 is formed by an epitaxial growth process known in the art. Insome embodiment, the doped semiconductor layer 110 is doped with adopant having an opposite (or different) type of conductivity from thatof the doped semiconductor layer 60. Thus, in the embodiment where thedoped semiconductor layer 60 is doped with an n-type dopant, the dopedsemiconductor layer 110 is doped with a p-type dopant. The dopedsemiconductor layer 110 includes a III-V group compound, which is agallium nitride compound in the illustrated embodiments. Thus, the dopedsemiconductor layer 110 may also be referred to as a doped galliumnitride layer. In some embodiments, the doped semiconductor layer 110has a thickness that is in a range from about 150 nm to about 200 nm.

A core portion of the LED 30 is created by the disposition of the MQWlayer 80 between the doped layers 60 and 110. When an electrical voltage(or electrical charge) is applied to the doped layers of the LED 30, theMQW layer 80 emits radiation such as light. The color of the lightemitted by the MQW layer 80 corresponds to the wavelength of theradiation. The radiation may be visible, such as blue light, orinvisible, such as ultraviolet (UV) light. The wavelength of the light(and hence the color of the light) may be tuned by varying thecomposition and structure of the materials that make up the MQW layer80.

As discussed above, existing MQWs may have inadequate electron-holerecombination rates. As a result, output power for existing LEDs may below, and there may be a large efficiency droop as well. To overcomethese problems plaguing existing LEDs, the LED 30 of the presentdisclosure utilizes graded thicknesses (i.e., vertical dimensions inFIG. 1) for the barrier layers 81-87 in the MQW layer 80. In moredetail, the active layers 91-96 have substantially constant or uniformthicknesses. That is, the respective thicknesses for the active layers91-96 do not vary much from one another. In some embodiments, thethicknesses for the active layers 81-87 are within a range from about 2nm to about 3.5 nm. On the other hand, according to various aspects ofthe present disclosure, the barrier layers 81-87 have thicknesses as afunction of their respective distances from the p-doped semiconductorlayer 110 (or from the electron blocking layer 100). The closer thebarrier layers 81-87 are to the p-doped semiconductor layer 110, thethinner they become. Thus, the barrier layer 81 is the thickest barrierlayer in the MQW layer 80 because it is located farthest from thep-doped semiconductor layer 110, the barrier layer 82 is thinner thanthe barrier layer 81 because it is located closer to the p-dopedsemiconductor layer 110, so on and so forth, and the barrier layer 87 isthe thinnest barrier layer in the MQW layer 80.

In some embodiments, the thicknesses for the barrier layers 81-87 varywithin 5%-15% between adjacent barrier layers. In other words, thebarrier layer 82 is thinner than the barrier layer 81 by about 5%-15%,the barrier layer 83 is thinner than the barrier layer 82 by about5%-15%, so on and so forth. For example, the thicknesses for the barrierlayers 81-87 may be about 12 nm, 11 nm, 10 nm, 9 nm, 8 nm, 7 nm, and 6nm. Of course, different thicknesses may be chosen for these barrierlayers depending on design requirements and manufacturing concerns inother embodiments.

It is also understood that the thicknesses of the barrier layers do notalways have to decrease as the barrier layers get closer to the p-dopedsemiconductor layer 110. It is envisioned that in some otherembodiments, the thicknesses between two or more adjacent barrier layersmay stay relative the same, as long as the overall trend is that thebarrier layers become thinner as they approach the p-doped semiconductorlayer 110.

The graded thicknesses for the barrier layers 81-87 improve a holeinjection rate, thereby improving electron-hole combination rates. Inmore detail, the decay of carrier concentration is a function ofdistance. In the case of holes, its concentration is the greatest nearthe p-doped semiconductor layer 110 and the lowest near the n-dopedsemiconductor layer 60. The decay of the hole concentration may beexponential, that is, the decay of the hole concentration will speed updrastically the farther it gets from the p-doped semiconductor layer110. As a result, for conventional LEDs, holes cannot be easily moved(i.e., low mobility), especially under high current conditions. Thisresults in a large efficiency droop.

According to the present disclosure, the barrier layers 81-87 havedecreasing thicknesses from the n-doped semiconductor layer 60 to thep-doped semiconductor layer 110. Such graded thickness pattern for thebarrier layers 81-87 means that the holes can now travel more easilyfrom the p-doped semiconductor layer 110 toward the n-dopedsemiconductor layer 60. As a result, substantially every layer of theMQW layer 80 has an improved hole concentration (and a better holeinjection rate) compared to conventional LED devices. The improvement inhole mobility and hole concentration in the MQW layer 80 leads to betterelectron-hole recombination rates, which reduces droop and increaseslight output.

In addition to having graded barrier layer thicknesses, the MQW layer 80of the present disclosure also has graded doping for at least a subsetof the barrier layers, so as to further enhance electron-holerecombination. In more detail, a subset of the barrier layers that arelocated the closest to the p-doped semiconductor layer 110 is doped witha p-type dopant, such as Magnesium (Mg). Choosing the number of thebarrier layers to be doped involves a trade-off analysis. On the onehand, doping the barrier layers causes the holes to be more spread outin the MQW layer 80, thereby improving the injection rate of holes. Thissuggests that the more barrier layers that are doped, the better theelectron-hole recombination rate will be.

On the other hand, doping the barrier layers may damage the activelayers 91-96 in the MQW layer 80, which adversely impacts the quality ofthe MQW layer 80. Therefore, it is important to choose an optimizednumber of doped barrier layers, so that the hole injection rate can besubstantially improved, and yet damage to the active layers 91-96 isstill minimal. In some embodiments, the subset of the barrier layersthat are doped is at least three when there are six or more pairs ofactive/barrier layers. Thus, the barrier layers 85, 86, and 87 (theclosest to the p-doped semiconductor layer 110 in terms of distance) aredoped with a p-type dopant such as Mg in the illustrated embodiments.

According to the various aspects of the present disclosure, the dopedbarrier layers 85, 86, and 87 also have graded doping concentrationlevels as a function of their respective distances from the p-dopedsemiconductor layer 110. The closer a barrier layer is to the p-dopedsemiconductor layer 110, the higher its doping concentration level.Thus, among the three doped barrier layers 94, 95, and 96, the barrierlayer 96 has the greatest doping concentration level, the barrier layer95 has a lower doping concentration level than the barrier layer 96, andthe barrier layer 94 has the lowest doping concentration level. In someembodiments, the doping concentration levels for the barrier layers 85,86, and 87 are 5×10¹⁷ ions/cm³, 4×10¹⁷ ions/cm³, and 3×10¹⁷ ions/cm³,respectively.

Due at least in part to the graded thicknesses for the barrier layers81-87, and to the subset of barrier layers 85-87 having graded dopingconcentration levels, the MQW layer 80 discussed herein has improvedelectron-hole recombination compared to traditional MQW layers,especially under high current conditions. Consequently, droop issuesplaguing traditional LEDs can be substantially alleviated by the LED 30,and the light output for the LED 30 can be improved as well.

Additional processes may be performed to complete the fabrication of theLED 30. For example, referring to FIG. 2, an electrically-conductivecontact layer 120 may be formed over the doped-semiconductor layer 110.A portion of the layer 60 is etched away so that a part of the dopedsemiconductor layer 60 is exposed. Metal contacts 130-131 may then beformed on the surface of the exposed doped semiconductor layer 60 and onthe surface of the contact layer 120, respectively. The metal contacts130-131 are formed by one or more deposition and patterning processes.The metal contacts 130-131 allow electrical access to the dopedsemiconductor layer 60 and to the doped semiconductor layer 110,respectively.

The LED 30 having the improved MQW layer 80 as illustrated in FIGS. 1-2above pertains to a horizontal LED. Similarly, a vertical LED may alsobe fabricated to incorporate the improved MQW layer 80. For example,FIG. 3 illustrates an example of such vertical LED 150. Similarcomponents in the vertical and horizontal LEDs are labeled the same forreasons of consistency and clarity.

Referring to FIG. 3, the vertical LED 150 has a submount 160. Thesubmount 160 contains a metal material in the illustrated embodiments.In other embodiments, the submount 160 may include a silicon substrate.In yet other embodiments, the submount 160 may include a Metal CorePrinted Circuit Board (MCPCB), a lead frame, or a ceramic material. Thedoped semiconductor layer 110 is disposed on the submount 160. In theembodiment shown, the doped semiconductor layer 110 includes p-dopedgallium nitride (p-GaN). The electron blocking layer 100 is disposed onthe doped semiconductor layer 110. The MQW layer 80 is disposed on theelectron blocking layer 100. The pre-strained layer 70 is disposed onthe MQW layer 80. The doped semiconductor layer 60 is disposed on thepre-strained layer 70. In the embodiment shown, the doped semiconductorlayer 60 includes n-doped gallium nitride (nGaN). The metal contact 131is disposed on the contact layer 120. Electrical access to the dopedlayers of the LED 150 can be gained through the metal contact 131 andthe submount 160.

To complete the fabrication of the horizontal LED 30 or the vertical LED150, additional processes such as dicing, packaging, and testingprocesses may also be performed, but they are not illustrated herein forthe sake of simplicity.

FIGS. 4-6 include plots showing example experimental results to helpillustrate the improvements offered by the MQW layer 80 discussed above.FIGS. 4A-4D are plots of hole concentration with respect to position(along the LED). In other words, the horizontal axes of FIGS. 4A-4Drepresent position along the LED (i.e., vertical positions across theLED in FIGS. 1-3), and the vertical axes of FIGS. 4A-4D represent holeconcentrations. Each of the FIGS. 4A-4D corresponds to a differentcurrent density—current densities at 20 A/cm², 35 A/cm², 70 A/cm², and100 A/cm², respectively. Each of the FIGS. 4A-4D also include three datasets: dataset 200 for sample A where the barrier layer thicknesses areconstant (similar to traditional devices and used as a reference datasetherein), dataset 201 for sample B where the barrier layer thicknessesare graded but in an opposite manner as taught by the present disclosure(i.e., the barrier layers become thicker the closer they are to thep-doped semiconductor layer), and dataset 202 for sample C where thebarrier layer thicknesses are graded according to embodiments of thepresent disclosure.

As can be seen in FIGS. 4A-4D, the dataset 202 (an embodiment of thepresent disclosure) spans across significantly greater positions alongthe X-axis than either the dataset 200 or 201. This indicates that thehole concentration for the MQW layer of the present disclosure is muchmore spread out, whereas the hole concentration for conventional LEDdevices tends to be congregated in a small region—near the p-dopedsemiconductor layer. The above holds true regardless of the currentdensity, as the hole concentrations for the dataset 202 span across morepositions along the X-axis than the datasets 200-201 even as the currentdensities increase from FIG. 4A to FIG. 4D. Therefore, the experimentalresults illustrated in FIGS. 4A-4D confirm that the MQW layer of thepresent disclosure offers a better hole injection rate than forconventional devices.

FIGS. 5A-5D show experimental results of carrier recombinationdistribution with respect to position (along the LED). Similar to FIGS.4A-4D, FIGS. 5A-5D are associated with different current densities. Inaddition, the datasets 200, 201, and 202 discussed above are in FIGS.4A-4D are graphed in FIGS. 5A-5D. And as is the case in for holeconcentration (as shown in FIGS. 4A-4D), the carrier recombinationdistribution plots shown in FIGS. 5A-5D indicate that the dataset 202(i.e., dataset representing an embodiment of the present disclosure)spans across more regions of the LED, rather than being congregatedmostly near the p-doped semiconductor layer. In other words, FIGS. 5A-5Dshow that carrier recombination of the present disclosure has animproved distribution over traditional devices, which leads to highoutput power.

FIG. 6 is a plot of quantum efficiency versus current density toillustrate the droop rate improvement offered by the present disclosure.In more detail, the X-axis of FIG. 6 represents current density, and theY-axis of FIG. 6 represents quantum efficiency. Once again, shown inFIG. 6 are dataset 200 (sample A where the barrier layer thicknesses areconstant), dataset 201 (sample B where the barrier layer thicknesses aregraded but in an opposite manner as taught by the present disclosure),and dataset 202 (sample C where the barrier layer thicknesses are gradedaccording to embodiments of the present disclosure). For datasets 200and 201, it can be seen that as current density begins to increase, thequantum efficiency reaches a peak and begins to drop. This is referredto as droop or droop rate. The more pronounced the droop, the worse theperformance of the LED. In comparison, the quantum efficiency decline ascurrent increases is much smaller for the dataset 202, meaning that thepresent disclosure offers a significantly improved droop rate (i.e.,smaller droop) compared to traditional LED devices. Therefore, thepresent disclosure leads to performance enhancements.

It is understood that FIGS. 4-6 are merely example experimental results.Other experimental results may vary somewhat from those shown in FIGS.4-6 without departing from the spirit and the scope of the presentdisclosure.

The LED 30 having the improved MQW layer as discussed above may beimplemented as a part of a lighting apparatus. For example, the LED 30may be implemented as a part of a LED-based lighting instrument 300, asimplified cross-sectional view of which is shown in FIG. 7. Theembodiment of the LED-based lighting instrument 300 shown in FIG. 7includes a plurality of LED dies. In other embodiments, the lightinginstrument 300 may include a single LED die.

As discussed above, the LED dies include an n-doped III-V group compoundlayer, a p-doped III-V group compound layer, and a MQW layer disposedbetween the n-doped and p-doped III-V compound layers. The MQW layer hasgraded thicknesses for its barrier layers and/or a subset of dopedbarrier layers having graded doping concentration levels close to thep-doped III-V compound layer. Due to these improvements in the MQWlayer, the LED dies offer less droop and better light output performancecompared to traditional LED dies.

In some embodiments, the LED dies 30 each have a phosphor layer coatedthereon. The phosphor layer may include either phosphorescent materialsand/or fluorescent materials. The phosphor layer may be coated on thesurfaces of the LED dies 30 in a concentrated viscous fluid medium(e.g., liquid glue). As the viscous liquid sets or cures, the phosphormaterial becomes a part of the LED package. In practical LEDapplications, the phosphor layer may be used to transform the color ofthe light emitted by an LED dies 30. For example, the phosphor layer cantransform a blue light emitted by an LED die 30 into a differentwavelength light. By changing the material composition of the phosphorlayer, the desired light color emitted by the LED die 30 may beachieved.

The LED dies 30 are mounted on a substrate 320. In some embodiments, thesubstrate 320 includes a Metal Core Printed Circuit Board (MCPCB). TheMCPCB includes a metal base that may be made of aluminum (or an alloythereof). The MCPCB also includes a thermally conductive butelectrically insulating dielectric layer disposed on the metal base. TheMCPCB may also include a thin metal layer made of copper that isdisposed on the dielectric layer. In alternative embodiments, thesubstrate 320 may include other suitable thermally conductivestructures. The substrate 320 may or may not contain active circuitryand may also be used to establish interconnections. In some otherembodiments, the substrate 320 includes a lead frame, a ceramic or asilicon substrate.

The lighting instrument 300 includes a diffuser cap 350. The diffusercap 350 provides a cover for the LED dies 30 therebelow. Stateddifferently, the LED dies 30 are encapsulated by the diffuser cap 350and the substrate 320 collectively. In some embodiments, the diffusercap 350 has a curved surface or profile. In some embodiments, the curvedsurface may substantially follow the contours of a semicircle, so thateach beam of light emitted by the LED dies 30 may reach the surface ofthe diffuser cap 350 at a substantially right incident angle, forexample, within a few degrees of 90 degrees. The curved shape of thediffuser cap 350 helps reduce Total Internal Reflection (TIR) of thelight emitted by the LED dies 30.

The diffuser cap 350 may have a textured surface. For example, thetextured surface may be roughened, or may contain a plurality of smallpatterns such as polygons or circles. Such textured surface helpsscatter the light emitted by the LED dies 30 so as to make the lightdistribution more uniform. In some embodiments, the diffuser cap 350 iscoated with a diffuser layer containing diffuser particles.

In some embodiments, a space 360 between the LED dies 30 and thediffuser cap 350 is filled by air. In other embodiments, the space 360may be filled by an optical-grade silicone-based adhesive material, alsoreferred to as an optical gel. Phosphor particles may be mixed withinthe optical gel in that embodiment so as to further diffuse lightemitted by the LED dies 30.

Though the illustrated embodiment shows all of the LED dies 30 beingencapsulated within a single diffuser cap 350, it is understood that aplurality of diffuser caps may be used in other embodiments. Forexample, each of the LED dies 30 may be encapsulated within a respectiveone of the plurality of diffuser caps.

The lighting instrument 300 may also optionally include a reflectivestructure 370. The reflective structure 370 may be mounted on thesubstrate 320. In some embodiments, the reflective structure is shapedlike a cup, and thus it may also be referred to as a reflector cup. Thereflective structure encircles or surrounds the LED dies 30 and thediffuser cap 350 in 360 degrees from a top view. From the top view, thereflective structure 370 may have a circular profile, a beehive-likehexagonal profile, or another suitable cellular profile encircling thediffuser cap 350. In some embodiments, the LED dies 30 and the diffusercap 350 are situated near a bottom portion of the reflective structure370. Alternatively stated, the top or upper opening of the reflectivestructure 370 is located above or over the LED dies 30 and the diffusercap 350.

The reflective structure 370 is operable to reflect light thatpropagates out of the diffuser cap 350. In some embodiments, the innersurface of reflective structure 370 is coated with a reflective film,such as aluminum, silver, or alloys thereof. It is understood that thesurface of the sidewalls of the reflective structure 370 may be texturedin some embodiments, in a manner similar to the textured surface of thediffuser cap 350. Hence, the reflective structure 370 is operable toperform further scattering of the light emitted by the LED dies 30,which reduces glare of the light output of the lighting instrument 300and makes the light output friendlier to the human eye. In someembodiments, the sidewalls of the reflective structure 370 have a slopedor tapered profile. The tapered profile of the reflective structure 370enhances the light reflection efficiency of the reflective structure370.

The lighting instrument 300 includes a thermal dissipation structure380, also referred to as a heat sink 380. The heat sink 380 is thermallycoupled to the LED dies 30 (which generate heat during operation)through the substrate 320. In other words, the heat sink 380 is attachedto the substrate 320, or the substrate 320 is located on a surface ofthe heat sink 380. The heat sink 380 is configured to facilitate heatdissipation to the ambient atmosphere. The heat sink 380 contains athermally conductive material, such as a metal material. The shape andgeometries of the heat sink 380 are designed to provide a framework fora familiar light bulb while at the same time spreading or directing heataway from the LED dies 30. To enhance heat transfer, the heat sink 380may have a plurality of fins 390 that protrude outwardly from a body ofthe heat sink 380. The fins 390 may have substantial surface areaexposed to ambient atmosphere to facilitate heat transfer.

FIG. 8 illustrates a simplified diagrammatic view of a lighting module400 that includes some embodiments of the lighting instrument 300discussed above. The lighting module 400 has a base 410, a body 420attached to the base 410, and a lamp 430 attached to the body 420. Insome embodiments, the lamp 430 is a down lamp (or a down light lightingmodule). The lamp 430 includes the lighting instrument 300 discussedabove with reference to FIG. 7. The lamp 430 is operable to efficientlyproject light beams 440. In addition, the lamp 430 can offer greaterdurability and longer lifetime compared to traditional incandescentlamps.

Though the MQW improvements discussed above are illustrated using LEDsas an example, it is understood that similar MQW layers may also beimplemented for laser diodes (LDs). FIG. 9 illustrates a simplifiedcross-sectional side view of an embodiment of the LD 500 according tovarious aspects of the present disclosure.

The LD 500 includes a substrate 510, which is a silicon substrate in theembodiment shown. A III-V group compound layer 520 is formed over thesubstrate 510. In some embodiments, the III-V compound layer 520includes AlN. Another III-V compound layer 530 is formed over the III-Vcompound layer 510. In some embodiments, the III-V compound layer 530includes a plurality of sub-layers, for example AlGaN sub-layers. Thethicknesses for these sub-layers may increase, and the aluminum contentfor these sub-layers may decrease, as the sub-layer go up (i.e., fartheraway from the substrate 510).

A III-V compound epi layer 540 is then formed over the III-V compoundlayer 530. In some embodiments, the III-V compound epi layer 540 mayinclude GaN. Thereafter, an AlN layer or an AlGaN layer 550 is formedover the III-V compound epi layer 540. Another III-V compound epi layer560 is then formed over the AlN or AlGaN layer 550.

An n-doped III-V compound layer 570 is then formed over the III-Vcompound epi layer 560. In some embodiments, the n-doped III-V compoundlayer 570 includes n-type doped GaN. A plurality of other layers 580 maybe formed over the n-doped III-V compound layer 570, for exampleincluding an n-doped InGaN layer, a cladding layer containing n-dopedInAlGaN, and a guiding layer containing n-doped InGaN.

Thereafter, a MQW layer such as the MQW layer 80 of FIGS. 1-3 may beformed over the layer 580 (and over the n-doped III-V compound layer570). As discussed above, the MQW layer includes interleaving barrierlayers and active layers, where the barrier layers have gradedthicknesses, and where a subset of the barrier layers are doped with agraded doping concentration.

An electron blocking layer 590 is formed over the MQW layer 80. In someembodiments, the electron blocking layer 590 includes p-doped InAlGaN.Thereafter, a guiding layer 600 is formed over the electron blockinglayer 590. In some embodiments, the guiding layer 600 includes a p-dopedInGaN. A cladding layer 610 is then formed over the guiding layer. Insome embodiments, the cladding layer 610 includes a p-doped InAlGaN. Ap-doped III-V compound layer 620 is then formed over the cladding layer610. In some embodiments, the p-doped III-V compound layer 570 includesp-type doped GaN.

The various layers of the LD 500 discussed above and shown in FIG. 9 aremerely example layers. Other LDs may incorporate different layersdepending on the design needs.

FIG. 10 is a flowchart illustrating a simplified method 700 offabricating a MQW layer for a photonic device according to the variousaspects of the present disclosure. The photonic device may be ahorizontal LED, a vertical LED, or an LD.

The method 700 includes a step 710, in which a first doped semiconductorlayer is formed. The first doped semiconductor layer may be formed overa substrate, such as a sapphire substrate for example. In someembodiments, the first doped semiconductor layer includes an n-dopedIII-V compound material, such as n-doped GaN.

The method 700 includes a step 720, in which a MQW layer is formed overthe first doped semiconductor layer. The MQW layer includes a pluralityof interleaving barrier layers and active layers. The barrier layershave graded thicknesses in a manner so that the thicknesses decrease asthe barrier layer is located farther and farther away from the firstdoped semiconductor layer. A subset of the barrier layers of the MQWlayer is also doped with a p-type dopant such as Mg. In someembodiments, the three barrier layers farthest away from the first dopedsemiconductor layer are the doped barrier layers. The doped barrierlayers also have graded doping concentration levels such that the dopingconcentration levels increase as the barrier layer is located fartherand farther away from the first doped semiconductor layer.

The method 700 includes a step 730, in which a second dopedsemiconductor layer is formed over the MQW layer. In some embodiments,the second doped semiconductor layer includes a p-doped III-V compoundmaterial, such as p-doped GaN.

Additional processes may be performed before, during, or after theblocks 710-730 discussed herein to complete the fabrication of thephotonic device. These other processes are not discussed in detailherein for reasons of simplicity.

One aspect the present disclosure involves a photonic device. Thephotonic device includes: an n-type III-V group layer disposed over asubstrate; a p-type III-V group layer disposed over the n-type layer; aquantum well disposed between the n-type III-V group layer and thep-type III-V group layer; wherein: the quantum well includes a pluralityof active layers interleaved with a plurality of barrier layers; theactive layers have substantially uniform thicknesses; and a respectivethickness of each barrier layer is a function of its location withrespect to the p-type III-V group layer.

In some embodiments, the function is such that the thicknesses of thebarrier layers decrease the closer the barrier layer gets to the p-typeIII-V group layer.

In some embodiments, a thickness variation between adjacent barrierlayers is in a range from about 5% to about 15%.

In some embodiments, the n-type III-V group layer and the p-type HI-Vgroup layer include n-doped gallium nitride (n-GaN) and p-doped galliumnitride (p-GaN), respectively; the active layers contain indium galliumnitride (InGaN); and the barriers layers contain gallium nitride (GaN).

In some embodiments, the photonic device includes one of: alight-emitting diode (LED) and a laser diode (LD).

In some embodiments, the photonic device includes a lighting modulehaving a plurality of dies, and wherein the n-type and p-type III-Vgroup layers and the quantum well are implemented in each of the dies.

In some embodiments, a subset of the barrier layers are doped with ap-type dopant. In some embodiments, the subset of the barrier layersinclude at least three barrier layers that are located the closest tothe p-type III-V group layer. Each of the barriers in the subset has arespective doping concentration level that is a function of its locationwith respect to the p-type III-V group layer. In some embodiments, thedoping concentration levels of the barriers in the subset increase thecloser the barrier layer gets to the p-type III-V group layer.

Another aspect the present disclosure involves a photonic device. Thephotonic device includes: an n-type III-V group layer disposed over asubstrate; a p-type III-V group layer disposed over the n-type layer; aquantum well disposed between the n-type III-V group layer and thep-type III-V group layer; wherein: the quantum well includes a pluralityof active layers interleaved with a plurality of barrier layers; atleast some of the barrier layers in the quantum well are doped with ap-type dopant; the doping concentration for the barrier layers that aredoped increases as distances between the barrier layer and the p-typeIII-V group layer become smaller.

In some embodiments, the barrier layers that are doped include at leastthree barrier layers that have shortest distances from the p-type III-Vgroup layer.

In some embodiments, the active layers have substantially uniformthicknesses; and the barrier layers become thinner as they get closer tothe p-type III-V group layer.

In some embodiments, the n-type III-V group layer and the p-type III-Vgroup layer include n-doped gallium nitride (n-GaN) and p-doped galliumnitride (p-GaN), respectively; the active layers contain indium galliumnitride (InGaN); and the barriers layers contain gallium nitride (GaN).

In some embodiments, the photonic device includes one of: alight-emitting diode (LED) and a laser diode (LD).

In some embodiments, the photonic device includes a lighting instrumentwhose light source includes one or more light-emitting dies, and whereinthe n-type and p-type III-V group layers and the quantum well areimplemented in each of the one or more light-emitting dies.

Yet another aspect the present disclosure involves an illuminationapparatus. The illumination apparatus includes: an n-doped semiconductorcompound layer; a p-doped semiconductor compound layer spaced apart fromthe n-doped semiconductor compound layer; and a multiple-quantum-well(MQW) disposed between the first semiconductor compound layer and thesecond semiconductor compound layer, the MQW including a plurality ofalternating first and second layers; wherein: the first layers of theMQW have substantially uniform thicknesses; the second layers havegraded thicknesses with respect to distances from the p-dopedsemiconductor compound layer; and a subset of the second layers locatedmost adjacent to the p-doped semiconductor compound layer is doped witha p-type dopant and have graded doping concentration levels that varywith respect to distances from the p-doped semiconductor layer.

In some embodiments, the n-doped semiconductor compound layer includesn-doped gallium nitride (n-GaN); the p-doped semiconductor compoundlayer includes p-doped gallium nitride (p-GaN); the first layer of theMQW includes indium gallium nitride (InGaN); and the second layer of theMQW includes gallium nitride (GaN).

In some embodiments, the second layers become thinner as they approachthe p-doped semiconductor compound layer; and the doping concentrationlevels for the subset of doped second layers become greater as theyapproach the p-doped semiconductor compound layer.

In some embodiments, the apparatus is a lighting module having alight-emitting diode (LED) or a laser diode (LD) as its light source;and the n-doped semiconductor compound layer, the p-doped semiconductorcompound layer, and the MQW are disposed within the LED or the LD.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claim is:
 1. A photonic device, comprising: a first-type III-Vgroup layer; a second-type III-V group layer formed on the first-typeIII-V group layer; and a multi-quantum well layer disposed between thefirst-type III-V group layer and the second-type III-V group layer;wherein: the multi-quantum well layer comprises a plurality of activelayers interleaved with a plurality of barrier layers such that eachbarrier layer is separated from adjacent barrier layers by a respectiveone of the active layer; a material of each barrier layer comprisessemiconductor compound devoid of Al element; the barrier layerscomprises a first group layers between the first-type III-V group layerand the second-type III-V group layer and a second group layers betweenthe second-type III-V group layer and the first group layers, and athickness of each barrier layer of the first group layers is greaterthan that of each barrier layer of the second group layers; and thebarrier layers of the first group layers comprise uniform thickness. 2.The photonic device of claim 1, wherein the barrier layers of the secondgroup layers comprise uniform thickness.
 3. The photonic device of claim2, wherein number of the barrier layers of the first group layerscomprises two or more.
 4. The photonic device of claim 3, wherein adoping concentration level of one of the barrier layers is equal to orgreater than 3×10¹⁷ ions/cm³.
 5. The photonic device of claim 1, whereinone of the barrier layers of the first group layers comprises athickness greater than 10 nm, and one of the barrier layers of thesecond group layers comprises a thickness less than 10 nm.
 6. Thephotonic device of claim 1, wherein the photonic device compriseslight-emitting diode (LED).
 7. The photonic device of claim 6, whereinthe multi-quantum well layer emits visible light or invisible light. 8.The photonic device of claim 7, wherein the visible light comprises bluelight and the invisible light comprises UV light.
 9. The photonic deviceof claim 6, further comprising a lamp in which the LED is located. 10.The photonic device of claim 1, wherein the active layers compriseuniform thickness.
 11. The photonic device of claim 1, furthercomprising a substrate formed under the first-type III-V group layer.12. The photonic device of claim 1, wherein the active layers containindium gallium nitride (InGaN); and the barriers layers contain galliumnitride (GaN).
 13. The photonic device of claim 1, wherein thefirst-type III-V group layer comprises n-type; and the second-type III-Vgroup layer comprises p-type.
 14. The photonic device of claim 13,wherein the first-type III-V group layer comprises n-doped galliumnitride (n-GaN); and the second-type III-V group layer comprises p-dopedgallium nitride (p-GaN).
 15. The photonic device of claim 1, furthercomprising an electron-blocking layer formed between the multi-quantumwell layer and the second-type III-V group layer.
 16. The photonicdevice of claim 1, wherein a barrier layer closest to the first-typesemiconductor layer is within the first group layers, and anotherbarrier closest to the second-type semiconductor layer is within thesecond group layers.
 17. The photonic device of claim 1, furthercomprising a pre-strained layer formed between the first-type III-Vgroup layer and the multi-quantum well layer.
 18. The photonic device ofclaim 17, wherein the pre-strained layer comprises a super-latticestructure containing In_(x)Ga_(1-x)N and GaN layers.
 19. The photonicdevice of claim 1, wherein a thickness of each active layer is between 2nm to 3.5 nm.
 20. The photonic device of claim 1, wherein one of thefirst group layers and the second group layers is doped with a dopant.